1. Field of the Invention
The present invention releates to a catalyst for exhaust gas purification capable of effectively purifying the harmful substances present in exhaust gas from engine, particularly the hydrocarbons generated in a large amount during the cold start of engine, as well as to an exhaust gas purification system using the catalyst.
2. Description of Related Art
Active researches and developments have heretofore been made on catalysts for exhaust gas purification, used for purifying the harmful substances present in exhaust gas from automobile engine, such as nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC) and the like. In recent years, as the regulation on exhaust gas has become stricter, the purification of HC during engine start (hereinafter referred to as xe2x80x9cduring the cold startxe2x80x9d) has become an important technical task.
The reason is as follows. During the cold start when the temperature of exhaust gas is low, the catalyst disposed in the exhaust pipe of engine does not reach its light-off temperature and has a low exhaust gas purifiability; moreover, during this period as compared with the period of continuous operation of engine, a large amount of HC is discharged from the engine; as a result, the HC discharged during the cold start occupies a large proportion of the total harmful substances discharged from the engine.
As one means for achieving the above-mentioned technical task, a Pd-only catalyst is known which uses, as the catalyst component, Pd having an excellent low-temperature light-off property. The Pd-only catalyst is produced ordinarily by forming, on a monolithic carrier, a catalyst layer comprising and a heat-resistant inorganic oxide (e.g. Al2O3) and Pd loaded thereon in a dispersed state.
In purification of HC during the cold start, it is important to activate the catalyst used, as early as possible. The effective means for achieving it includes use of a catalyst of small heat capacity. One parameter for determining the heat capacity of catalyst is the thickness of catalyst layer loaded on monolithic carrier. There is known no conventional Pd-only catalyst in which the thickness of catalyst layer is optimized for the above purpose.
Incidentally, description is made on the thickness of catalyst layer of exhaust gas purification catalyst, in JP-A-5-293376, JP-A-5-293384 and JP-A-6-233918. These descriptions are not for Pd-only catalysts.
In view of the above situation, the present invention is intended to provide a Pd-only catalyst wherein the thickness of catalyst layer is optimized in order to reduce the heat capacity to achieve early catalyst activation and which is improved in HC purifiability during the cold start.
According to the present invention there is provided a catalyst for exhaust gas purification, comprising (1) a monolithic carrier having passages and (2) palladium-loaded alumina obtained by immersing alumina in a palladium solution and then firing the resulting alumina and loaded on the monolithic carrier as a catalyst layer, in which catalyst the catalyst layer loaded on the partition walls of the passages of the monolithic carrier has a thickness of 5 to 100 xcexcm and the palladium/alumina (Pd/Al2O3) weight ratio in the catalyst layer is 0.03 to 0.30.
According to the present invention there is also provided an exhaust gas purification system disposed in an exhaust pipe of internal combustion engine, comprising (a) the above-mentioned catalyst for exhaust gas purification and (b) an adsorbent disposed upstream of the catalyst (a) in the flow direction of the exhaust gas discharged from the engine, which is obtained by loading an adsorbent material having a hydrocarbon adsorptivity, on a monolithic carrier.
In the catalyst layer loaded on a monolithic carrier having passages, its thickness is not uniform at all the sites of the partion walls of the passages and, when the catalyst layer is formed according to an ordinary process, is larger at and around the intersections of the partion walls. Therefore, in the present invention, xe2x80x9cthe thickness of catalyst layerxe2x80x9d refers, when the monolithic carrier has passages 22 having a polygonal sectional shape as shown in FIG. 1(a), to the thickness t of catalyst layer 26 (formed on partion wall 24) at the midpoint of partion wall 24 between the intersections A and B of partion walls 24 forming each passage 22; and when the monolithic carrier has passages 28 formed by placing a corrugated metallic plate 32 on a flat metallic plate 30, as shown in FIG. 1(b), to the thickness t of catalyst layer 34 (formed on partion wall) at the midpoint between the intersections C nd D of flat plate 30 and corrugated plate 32.